The Application Of Capillary Electrophoretic On-line Preconcentration In Some Weak Acidic Materials Analysis

Capillary electrophoresis(CE) is a kind of separation technology developed in recent years. Compared with other methods such as Gas Chromatography(GC), High Performance Liquid Chromatography(HPLC), CE has the advantages of high resolving power, low solvent consumption, rapidity and simple sample pretreatment. CE has become one of the most powerful method in separation science. But the application of CE is limited by its low sensitivity due to the small inner dimension of the capillary, which results in the small volume and short optical path. In order to solve the problem of low sensitivity and make it more suitable to analyze trace constituents, a series of capillary electrophoretic on-line sample preconcentration methods have been developed. These on-line preconcentration methods are based on migration changes of analytes in the sample zone and in background eletrolytes zone or through association with moving additives such as surfactants, as well as various combination of the above approaches. Water, fruit(apple juice) and traditional Chinese medicine(Solidaginis) were selected as sample to verify capillary electrophoretic on-line sample preconcentration schemes. The mechanisms of Field-Amplified Sample Injection(FASI), Electrokinetic Supercharging(EKS) and Large Volume Sample Stacking with Polarity Switching-Acid Barrage Stacking(LVSSPS-ABS) have been discussed. At last, the applicability of the developed methods was demonstrated.Objective:1 To build an on-line preconcentration method to analyze 2-nitrobenzoic acid, 3-nitrobenzoic acid and 4-nitrobenzoic acid in water by capillary electrophoresis.2 To build an on-line preconcentration method to analyze maleic acid and fumaric acid in apple juice by capillary electrophoresis.3 To build an on-line preconcentration method to analyze rutin, chlorogenic acid and quercetin in solidaginis sample.Methods: 1 A method of ?-cyclodextrin(?-CD) modified capillary zone electrophoresis with field-amplified sample injection(FASI) has been developed to determine three aromatic acids in water. The effects of buffer p H and concentration, ?-CD concentration and sample injection time were investigated. The optimized conditions were: the buffer solution of 30mmol/L citric acid(p H 3.5, adjusted with 30m M disodium hydrogen phosphate, ?-CD10 mmol/L) and sample injection time of 40 sat-5kV.2 A method has been developed to determine maleic acid and fumaric acid by nonaqueous capillary electrophoresis after on-line preconcentration with electrokinetic supercharging(EKS). Under the optimum conditions: The background electrolyte of 30mmol/L Tris-HOAc methanol solution, pH7.9, hydrodynamic injection of 20 mmol/L NH4Cl(3447Pa, 12s) as leader, electrokinetic injection of the sample(-10 kV, 110s), hydrodynamic injection of 20 mmol/L 4-nitrophenol(3447 Pa, 18 s) as terminator and then a voltage of 25 k V for separation(-25 k V).3 A method for the determination of rutin, chlorogenic acid and quercetin by large volume sample stacking with polarity switching and acid barrage stacking(LVSSPS-ABS) has been developed. The optimum condition were: background electrolyte of 30 mmol/L Na2B2O7 of p H 9.25, hydrodynamic injection of the sample(60 s, 5 psi), then applied voltage of-25 k V, and then hydrodynamic injection of 0.15 mol/L HAc(18 s, 3447 Pa), and at last separation with 25 k V.Results: 1 Compared with conventional hydrodynamic sample injection, sensitivity enhancement factors for 2-nitrobenzoic, 3-nitrobenzoic acid and 4-nitrobenzoic were 113, 76 and 62, respectively. The detection limits were in the range of 24~34 ng/ml. This method was applied to the analysis of aromatic acids in water sample, recoveries from 54.8% to 83.2% have been achieved.2 The sensitivity enhancement factors and detection limits were 2278, 5.4 ng/m L for maleic acid and 1051, 7.9 ng/m L for fumaric acid. The developed method was applied to analyze apple juice and recovery experiments were also carried out. The recoveries for maleic acid and fumaric acid were 64.8% and 71.2%, respectively.3 Compared with conventional hydrodynamic injection, the sensitivity has been enhanced between 333 to 506 times and the method is 3.6-5.3 times more sensitive than LVSSPS only. The applicability of developed method was demonstrated by the detection of analytes in aqueous extract of Solidginis.Conclusion:A series of capillary electrophoretic on-line preconcentration methods such as FASI, EKS and LVSSPS-ABS have been developed. These methods were applied to the analysis of weak acidic materals in water, apple juice and solidaginis. The sensivity of these developed methods are comparable to HPLC. So, it can become a referenced method in food and medicinal analysis.